Abstract
Type-I clathrates possess extremely low thermal conductivities, a property that makes them promising materials for thermoelectric applications. The incorporation of cerium into one such clathrate has recently been shown to lead to a drastic enhancement of the thermopower, another property determining the thermoelectric efficiency. Here we explore the mechanism of the incorporation of rare earth elements into type-I clathrates. Our investigation of the crystal growth and the composition of the phase Ba8–xRExTMySi46–y (RE = rare earth element; TM = Au, Pd, Pt) reveals that the RE content x is mainly governed by two factors, the free cage space and the electron balance.
Highlights
In order to make thermoelectric materials economically viable, a drastic increase in their efficiency should be achieved
Two approaches aim at increasing the thermoelectric efficiency: the maximization of the power factor and the minimization of the thermal conductivity.[1]
The interaction of heat carrying acoustic phonons propagating through the framework with strongly anharmonic vibrations of the encapsulated atoms results in low thermal conductivity.[2−4] The power factor has been maximized over the past few years by optimizing the charge carrier concentration through tuning the framework composition; the dimensionless thermoelectric figure of merit ZT characterizing the total efficiency reaches 1.63 for n-type Ba8Ga16−xGe30+x at 1100 K and 1.1 for p-type Ba8Ga16+xGe30−x at 900 K.5,6
Summary
In order to make thermoelectric materials economically viable, a drastic increase in their efficiency should be achieved. The RE content measured by EDX in clathrate phases shows a general tendency to decrease with atomic number, with two distinct anomalies at Eu and Yb (Figure 7a) This behavior reflects the well-known peculiarities of Eu and Yb in the lanthanide series which originate from their ability to exist in the divalent state (e.g., variation of the ionic radius, Figure 7b). The clear correlation between the solubility of the RE element in the clathrate phase and its ionic radius (Figure 7c) leads to the following picture: too much free space in the cage destabilizes the clathrate phase This is the reason why the trivalent Ce and La ions, that are smaller than divalent Ba preferentially occupy the 2a site in the small cages instead of the 6d sites in the larger cages.[8] The observed destabilization effect due to a large free volume is in agreement with previous observations. ■compared to the 3p orbital of Si plays a role
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